Field of the Invention
The present invention relates to techniques for anti-roll stabilization of watercraft.
The present invention has been developed in particular for stabilization of watercraft standing at anchor.
Description of the Related Art
One of the major causes of malaise on board a watercraft, both during navigation and at anchor, is the roll to which the watercraft is subjected on account of wave motion.
For this reason, stabilization systems are frequently used that comprise one or more stabilizing fins. The purpose of stabilizing fins is to increase the comfort on board by reducing considerably any motion of roll in every condition of use of the watercraft, both during navigation and at anchor.
For instance, through rotation of one or more pairs of fins, which are symmetrical with respect to the longitudinal axis, it is possible to create a moment on the watercraft that can be exploited for countering the moment generated by the wave motion and for reducing considerably roll.
In particular, during navigation the stabilizing fins exploit the phenomenon of lift for generating high stabilizing moments with a relatively exiguous surface of actuation. For instance, the documents Nos. GB 999 306, EP 0 754 618, GB 1 201 401 describe systems for anti-roll stabilization of watercraft during navigation.
Instead, at anchor it is not possible to exploit the lift but it is necessary to exploit the inertial forces (acceleration and deceleration) and the forces of viscous resistance (linked to the velocity of actuation of the fin) for generating the stabilizing moment. It may easily be understood that for stabilization at anchor a surface of actuation is necessary that is considerably greater than the one sufficient during navigation and that the aspect ratio of the fin has a major influence on efficiency. For instance, the document No. EP 1 577 210 describes such a system for anti-roll stabilization of watercraft standing at anchor in which the aspect ratio of the fin is modifiable.
FIG. 1 illustrates in this context a generic control scheme, in which a control system 10 controls operation of a system 20. In particular, the control system 10 comprises a control module 102 configured for generating a control signal u necessary for minimizing and/or annulling an error e. For instance, the error e can be determined in a block 100 as the difference between a reference signal r and a measurement signal y that indicates the state of the system 20.
In particular, in the case of the stabilizing fins, the system 20 comprises both the watercraft 202 and the system for actuation of the fins 200. Consequently, the control system 10 has the purpose of countering roll; i.e., the reference signal r is typically zero, the measurement signal y corresponds to a signal that indicates the roll of the watercraft ϑ, and the control signal u is the signal that drives the actuator of the fin.
Finally, the document No. EP 2 172 394 describes a system for anti-roll stabilization of watercraft in which an electric motor is used as actuator for the stabilizing fin. In particular, the control system controls movement of the fin as a function of the roll of the watercraft and of the angular position of the stabilizing fin; i.e., the state signal y also comprises a signal that indicates the angular position of the stabilizing fin.